Magnetic domains in a magnetic bubble film have been observed in different bias fields after application of pulsed bias fields. The experimental results indicate that the formation of multifingered domains during the rapid expansion of bubbles is usually accompanied by the generation of a large amount of vertical Bloch lines (VBL). The sign of the VBL has a definite relation to the strength of the bias fields applied. On application of a series of bias pulses which are not very strong, the motion of strip domain heads leads to the generation of a large amount of VBL in the domain walls. The VBL will disappear when the pulsed bias fields are strong enough.